mirror of
https://github.com/apache/httpd.git
synced 2025-10-31 19:10:37 +03:00
1bf05b9838
in particular, the non-english files, to make sure I did not screw anything up. They look fine to me. git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91112 13f79535-47bb-0310-9956-ffa450edef68
345 lines
14 KiB
HTML
345 lines
14 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
|
|
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
|
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|
<head>
|
|
<meta name="generator" content="HTML Tidy, see www.w3.org" />
|
|
|
|
<title>Dynamic Shared Object (DSO) support</title>
|
|
</head>
|
|
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
|
|
|
|
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
|
|
vlink="#000080" alink="#FF0000">
|
|
<!--#include virtual="header.html" -->
|
|
|
|
<h1 align="center">Dynamic Shared Object (DSO) Support</h1>
|
|
|
|
<p>The Apache HTTP Server is a modular program where the
|
|
administrator can choose the functionality to include in the
|
|
server by selecting a set of modules. The modules can be
|
|
statically compiled into the <code>httpd</code> binary when the
|
|
server is built. Alternatively, modules can be compiled as
|
|
Dynamic Shared Objects (DSOs) that exist separately from the
|
|
main <code>httpd</code> binary file. DSO modules may be
|
|
compiled at the time the server is built, or they may be
|
|
compiled and added at a later time using the Apache Extension
|
|
Tool (<a href="programs/apxs.html">apxs</a>).</p>
|
|
|
|
<p>This document describes how to use DSO modules as well as
|
|
the theory behind their use.</p>
|
|
|
|
<ul>
|
|
<li><a href="#implementation">Implementation</a></li>
|
|
|
|
<li><a href="#usage">Usage Summary</a></li>
|
|
|
|
<li><a href="#background">Background</a></li>
|
|
|
|
<li><a href="#advantages">Advantages and
|
|
Disadvantages</a></li>
|
|
</ul>
|
|
<hr />
|
|
|
|
<table border="1">
|
|
<tr>
|
|
<td valign="top"><strong>Related Modules</strong><br />
|
|
<br />
|
|
<a href="mod/mod_so.html">mod_so</a><br />
|
|
</td>
|
|
|
|
<td valign="top"><strong>Related Directives</strong><br />
|
|
<br />
|
|
<a href="mod/mod_so.html#loadmodule">LoadModule</a><br />
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<h2><a id="implementation"
|
|
name="implementation">Implementation</a></h2>
|
|
|
|
<p>The DSO support for loading individual Apache modules is
|
|
based on a module named <a
|
|
href="mod/mod_so.html"><code>mod_so.c</code></a> which must be
|
|
statically compiled into the Apache core. It is the only module
|
|
besides <code>core.c</code> which cannot be put into a DSO
|
|
itself. Practically all other distributed Apache modules then
|
|
can then be placed into a DSO by individually enabling the DSO
|
|
build for them via <code>configure</code>'s
|
|
<code>--enable-<i>module</i>=shared</code> option as disucussed
|
|
in the <a href="install.html">install documentation</a>. After
|
|
a module is compiled into a DSO named <code>mod_foo.so</code>
|
|
you can use <a href="mod/mod_so.html"><code>mod_so</code></a>'s
|
|
<a
|
|
href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
|
|
command in your <code>httpd.conf</code> file to load this
|
|
module at server startup or restart.</p>
|
|
|
|
<p>To simplify this creation of DSO files for Apache modules
|
|
(especially for third-party modules) a new support program
|
|
named <a href="programs/apxs.html">apxs</a> (<em>APache
|
|
eXtenSion</em>) is available. It can be used to build DSO based
|
|
modules <em>outside of</em> the Apache source tree. The idea is
|
|
simple: When installing Apache the <code>configure</code>'s
|
|
<code>make install</code> procedure installs the Apache C
|
|
header files and puts the platform-dependent compiler and
|
|
linker flags for building DSO files into the <code>apxs</code>
|
|
program. This way the user can use <code>apxs</code> to compile
|
|
his Apache module sources without the Apache distribution
|
|
source tree and without having to fiddle with the
|
|
platform-dependent compiler and linker flags for DSO
|
|
support.</p>
|
|
|
|
<h2><a id="usage" name="usage">Usage Summary</a></h2>
|
|
|
|
<p>To give you an overview of the DSO features of Apache 2.0,
|
|
here is a short and concise summary:</p>
|
|
|
|
<ol>
|
|
<li>
|
|
Build and install a <em>distributed</em> Apache module, say
|
|
<code>mod_foo.c</code>, into its own DSO
|
|
<code>mod_foo.so</code>:
|
|
|
|
<table bgcolor="#f0f0f0" cellpadding="10">
|
|
<tr>
|
|
<td>
|
|
<pre>
|
|
$ ./configure --prefix=/path/to/install
|
|
--enable-foo=shared
|
|
$ make install
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</li>
|
|
|
|
<li>
|
|
Build and install a <em>third-party</em> Apache module, say
|
|
<code>mod_foo.c</code>, into its own DSO
|
|
<code>mod_foo.so</code>:
|
|
|
|
<table bgcolor="#f0f0f0" cellpadding="10">
|
|
<tr>
|
|
<td>
|
|
<pre>
|
|
$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c
|
|
--enable-foo=shared
|
|
$ make install
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</li>
|
|
|
|
<li>
|
|
Configure Apache for <em>later installation</em> of shared
|
|
modules:
|
|
|
|
<table bgcolor="#f0f0f0" cellpadding="10">
|
|
<tr>
|
|
<td>
|
|
<pre>
|
|
$ ./configure --enable-so
|
|
$ make install
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</li>
|
|
|
|
<li>
|
|
Build and install a <em>third-party</em> Apache module, say
|
|
<code>mod_foo.c</code>, into its own DSO
|
|
<code>mod_foo.so</code> <em>outside of</em> the Apache
|
|
source tree using <a href="programs/apxs.html">apxs</a>:
|
|
|
|
<table bgcolor="#f0f0f0" cellpadding="10">
|
|
<tr>
|
|
<td>
|
|
<pre>
|
|
$ cd /path/to/3rdparty
|
|
$ apxs -c mod_foo.c
|
|
$ apxs -i -a -n foo mod_foo.so
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</li>
|
|
</ol>
|
|
|
|
<p>In all cases, once the shared module is compiled, you must
|
|
use a <a
|
|
href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
|
|
directive in <code>httpd.conf</code> to tell Apache to activate
|
|
the module.</p>
|
|
|
|
<h2><a id="background" name="background">Background</a></h2>
|
|
|
|
<p>On modern Unix derivatives there exists a nifty mechanism
|
|
usually called dynamic linking/loading of <em>Dynamic Shared
|
|
Objects</em> (DSO) which provides a way to build a piece of
|
|
program code in a special format for loading it at run-time
|
|
into the address space of an executable program.</p>
|
|
|
|
<p>This loading can usually be done in two ways: Automatically
|
|
by a system program called <code>ld.so</code> when an
|
|
executable program is started or manually from within the
|
|
executing program via a programmatic system interface to the
|
|
Unix loader through the system calls
|
|
<code>dlopen()/dlsym()</code>.</p>
|
|
|
|
<p>In the first way the DSO's are usually called <em>shared
|
|
libraries</em> or <em>DSO libraries</em> and named
|
|
<code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
|
|
reside in a system directory (usually <code>/usr/lib</code>)
|
|
and the link to the executable program is established at
|
|
build-time by specifying <code>-lfoo</code> to the linker
|
|
command. This hard-codes library references into the executable
|
|
program file so that at start-time the Unix loader is able to
|
|
locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
|
|
paths hard-coded via linker-options like <code>-R</code> or in
|
|
paths configured via the environment variable
|
|
<code>LD_LIBRARY_PATH</code>. It then resolves any (yet
|
|
unresolved) symbols in the executable program which are
|
|
available in the DSO.</p>
|
|
|
|
<p>Symbols in the executable program are usually not referenced
|
|
by the DSO (because it's a reusable library of general code)
|
|
and hence no further resolving has to be done. The executable
|
|
program has no need to do anything on its own to use the
|
|
symbols from the DSO because the complete resolving is done by
|
|
the Unix loader. (In fact, the code to invoke
|
|
<code>ld.so</code> is part of the run-time startup code which
|
|
is linked into every executable program which has been bound
|
|
non-static). The advantage of dynamic loading of common library
|
|
code is obvious: the library code needs to be stored only once,
|
|
in a system library like <code>libc.so</code>, saving disk
|
|
space for every program.</p>
|
|
|
|
<p>In the second way the DSO's are usually called <em>shared
|
|
objects</em> or <em>DSO files</em> and can be named with an
|
|
arbitrary extension (although the canonical name is
|
|
<code>foo.so</code>). These files usually stay inside a
|
|
program-specific directory and there is no automatically
|
|
established link to the executable program where they are used.
|
|
Instead the executable program manually loads the DSO at
|
|
run-time into its address space via <code>dlopen()</code>. At
|
|
this time no resolving of symbols from the DSO for the
|
|
executable program is done. But instead the Unix loader
|
|
automatically resolves any (yet unresolved) symbols in the DSO
|
|
from the set of symbols exported by the executable program and
|
|
its already loaded DSO libraries (especially all symbols from
|
|
the ubiquitous <code>libc.so</code>). This way the DSO gets
|
|
knowledge of the executable program's symbol set as if it had
|
|
been statically linked with it in the first place.</p>
|
|
|
|
<p>Finally, to take advantage of the DSO's API the executable
|
|
program has to resolve particular symbols from the DSO via
|
|
<code>dlsym()</code> for later use inside dispatch tables
|
|
<em>etc.</em> In other words: The executable program has to
|
|
manually resolve every symbol it needs to be able to use it.
|
|
The advantage of such a mechanism is that optional program
|
|
parts need not be loaded (and thus do not spend memory) until
|
|
they are needed by the program in question. When required,
|
|
these program parts can be loaded dynamically to extend the
|
|
base program's functionality.</p>
|
|
|
|
<p>Although this DSO mechanism sounds straightforward there is
|
|
at least one difficult step here: The resolving of symbols from
|
|
the executable program for the DSO when using a DSO to extend a
|
|
program (the second way). Why? Because "reverse resolving" DSO
|
|
symbols from the executable program's symbol set is against the
|
|
library design (where the library has no knowledge about the
|
|
programs it is used by) and is neither available under all
|
|
platforms nor standardized. In practice the executable
|
|
program's global symbols are often not re-exported and thus not
|
|
available for use in a DSO. Finding a way to force the linker
|
|
to export all global symbols is the main problem one has to
|
|
solve when using DSO for extending a program at run-time.</p>
|
|
|
|
<p>The shared library approach is the typical one, because it
|
|
is what the DSO mechanism was designed for, hence it is used
|
|
for nearly all types of libraries the operating system
|
|
provides. On the other hand using shared objects for extending
|
|
a program is not used by a lot of programs.</p>
|
|
|
|
<p>As of 1998 there are only a few software packages available
|
|
which use the DSO mechanism to actually extend their
|
|
functionality at run-time: Perl 5 (via its XS mechanism and the
|
|
DynaLoader module), Netscape Server, <em>etc.</em> Starting
|
|
with version 1.3, Apache joined the crew, because Apache
|
|
already uses a module concept to extend its functionality and
|
|
internally uses a dispatch-list-based approach to link external
|
|
modules into the Apache core functionality. So, Apache is
|
|
really predestined for using DSO to load its modules at
|
|
run-time.</p>
|
|
|
|
<h2><a id="advantages" name="advantages">Advantages and
|
|
Disadvantages</a></h2>
|
|
|
|
<p>The above DSO based features have the following
|
|
advantages:</p>
|
|
|
|
<ul>
|
|
<li>The server package is more flexible at run-time because
|
|
the actual server process can be assembled at run-time via <a
|
|
href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
|
|
<code>httpd.conf</code> configuration commands instead of
|
|
<code>configure</code> options at build-time. For instance
|
|
this way one is able to run different server instances
|
|
(standard & SSL version, minimalistic & powered up
|
|
version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
|
|
installation.</li>
|
|
|
|
<li>The server package can be easily extended with
|
|
third-party modules even after installation. This is at least
|
|
a great benefit for vendor package maintainers who can create
|
|
a Apache core package and additional packages containing
|
|
extensions like PHP3, mod_perl, mod_fastcgi,
|
|
<em>etc.</em></li>
|
|
|
|
<li>Easier Apache module prototyping because with the
|
|
DSO/<code>apxs</code> pair you can both work outside the
|
|
Apache source tree and only need an <code>apxs -i</code>
|
|
command followed by an <code>apachectl restart</code> to
|
|
bring a new version of your currently developed module into
|
|
the running Apache server.</li>
|
|
</ul>
|
|
|
|
<p>DSO has the following disadvantages:</p>
|
|
|
|
<ul>
|
|
<li>The DSO mechanism cannot be used on every platform
|
|
because not all operating systems support dynamic loading of
|
|
code into the address space of a program.</li>
|
|
|
|
<li>The server is approximately 20% slower at startup time
|
|
because of the symbol resolving overhead the Unix loader now
|
|
has to do.</li>
|
|
|
|
<li>The server is approximately 5% slower at execution time
|
|
under some platforms because position independent code (PIC)
|
|
sometimes needs complicated assembler tricks for relative
|
|
addressing which are not necessarily as fast as absolute
|
|
addressing.</li>
|
|
|
|
<li>Because DSO modules cannot be linked against other
|
|
DSO-based libraries (<code>ld -lfoo</code>) on all platforms
|
|
(for instance a.out-based platforms usually don't provide
|
|
this functionality while ELF-based platforms do) you cannot
|
|
use the DSO mechanism for all types of modules. Or in other
|
|
words, modules compiled as DSO files are restricted to only
|
|
use symbols from the Apache core, from the C library
|
|
(<code>libc</code>) and all other dynamic or static libraries
|
|
used by the Apache core, or from static library archives
|
|
(<code>libfoo.a</code>) containing position independent code.
|
|
The only chances to use other code is to either make sure the
|
|
Apache core itself already contains a reference to it or
|
|
loading the code yourself via <code>dlopen()</code>.</li>
|
|
</ul>
|
|
<!--#include virtual="footer.html" -->
|
|
</body>
|
|
</html>
|
|
|